std::is_function

Since C++11

Header: <type_traits>

std::is_function is a UnaryTypeTrait.

# Declarations

template< class T >
struct is_function;

(since C++11)

# Notes

std::is_function can be implemented in much simpler ways. Implementations similar to the following one are used by new versions of libc++, libstdc++ and MS STL:

The implementation shown below is for pedagogical purposes, since it exhibits the myriad kinds of function types.

# Example

#include <functional>
#include <type_traits>
 
int f();
static_assert(std::is_function_v<decltype(f)>);
 
static_assert(std::is_function_v<int(int)>);
static_assert(!std::is_function_v<int>);
static_assert(!std::is_function_v<decltype([]{})>);
static_assert(!std::is_function_v<std::function<void()>>);
 
struct O { void operator()() {} };
static_assert(std::is_function_v<O()>);
 
struct A
{
    static int foo();
    int fun() const&;
};
static_assert(!std::is_function_v<A>);
static_assert(std::is_function_v<decltype(A::foo)>);
static_assert(!std::is_function_v<decltype(&A::fun)>);
 
template<typename>
struct PM_traits {};
template<class T, class U>
struct PM_traits<U T::*> { using member_type = U; };
 
int main()
{
    using T = PM_traits<decltype(&A::fun)>::member_type; // T is int() const&
    static_assert(std::is_function_v<T>);
}

# See also